The preparation of polyisocyanates with uretdione structure by catalytic dimerization and optionally simultaneous trimerization of monomeric aliphatic or cycloaliphatic diisocyanates is known. A comprehensive overview of the industrially relevant dimerization processes of the prior art and of the catalysts and catalyst systems employed therein is in J. prakt. Chem. 336 (1994) 185–200.
Among the lightfast uretdione polyisocyanates there is great interest in the linear dimers, i.e. isocyanurate-group-free dimers, of cycloaliphatic diisocyanates in particular. These constitute preferred starting compounds in the preparation of blocking-agent-free polyurethane (PU) powder coating crosslinkers (e.g. EP-A 45 996, EP-A 639 598 or EP-A 669 353).
Whereas for the linear catalytic dimerization of aliphatic and/or cycloaliphatic diisocyanates containing at least one primarily attached (“primary”) isocyanate group, such as 1,6-diisocyanatohexane (HDI) or 1-isocyanato-3,3,5-trimethyl-5-isocyanatomethylcyclohexane (isophorone diisocyanate; IPDI), a variety of processes exist (e.g. EP-A 45 995, EP-A 317 744, EP-A 735 027 and EP-A 896 973), some of which are also employed on the industrial scale in the preparation of powder coating crosslinkers, there have to date been no disclosures of isocyanurate-group-free uretdione polyisocyanates formed from aliphatic and/or cycloaliphatic diisocyanates containing exclusively secondary and/or tertiary isocyanate groups. The activity of the customary dimerization catalysts with respect to such diisocyanates is either zero or so low that it is impossible to use them even in very high catalyst concentrations to prepare the corresponding dimers in anything other than a vanishingly small yield, if at all.
Although some of the cited publications on the catalytic dimerization of isocyanates, for example in EP-A 178 520, DE-A 34 20 114, EP-A 317 744 or EP-A 896 973, have mentioned, inter alia, diisocyanates such as 2,4′- and/or 4,4′-diisocyanatodicyclohexylmethane, 1,3- and/or 1,4-diisocyanatocyclohexane, 1,3-diisocyanatocyclobutane or 1,3-diisocyanato-2(4)-methylcyclohexane as possible starting compounds, there has never been any specific description of uretdione polyisocyanates formed from cycloaliphatic diisocyanates containing no primary isocyanate groups, owing to the lack of activity of the dimerization catalysts known to date.
It was therefore an object of the present invention to provide a novel process for preparing uretdione polyisocyanates from aliphatic and/or cycloaliphatic diisocyanates containing exclusively secondary and/or tertiary isocyanate groups which produces, using highly reactive and selective catalysts, products, ideally linear and preferably free of isocyanurate groups, such as are required as starting components for uretdione powder coating crosslinkers.